Rapidly increasing bandwidth demand has driven the development of optical transmitters capable of providing higher speed data transmissions. One strategy for developing optical transmitters with higher speed data transmissions is to rely on higher availability for electronic and optical components. However, by using multi-level formats, for example quadrature amplitude modulation (QAM), higher speed data transmissions may be achieved without increasing transmitter electrical bandwidth requirements. The recently developed digital signal processing (DSP) technologies made the scheme promising, in which the generation and detection of multi-level formats can be easily realized in DSP. Implementing multi-level formats for optical transmissions may lead to higher required optical signal-to-noise ratio (ROSNR) than binary formats, such as non-return-to-zero. Coherent detection may satisfy the ROSNR requirements, but may not be cost-effective due to additional components, for example hybrid mixers and local oscillators.